Revisions Affected

B, C

Details

This advisory applies to the EINVF32 and EISQRTF32 instructions. The expected results for these instructions are correct; however, the underflow (LUF) and overflow (LVF) flags are not. These flags are invalid and should not be used.

The LUF and LVF flags are not accessible using C code, so the overall impact of this advisory is expected to be small. If the user chooses to use these flags (for example, when coding a time-critical algorithm) in assembly as part of a mixed C/ASM project, the user will need to disable interrupts around the assembly code using the flags, and also preserve the flags through any use of EINVF32 or EISQRTF32 instructions.

Workarounds

There is no workaround for using these flags in C code, and they should be considered invalid for the reasons presented under NOTES ON COMPILER AND TOOLS USAGE.

The workaround shown here provides a way to preserve the LVF, LUF flags across the use of EISQRTF32 and EINVF32 in assembly-only code.

Do not rely on the LUF and LVF flags to catch underflow/overflow conditions resulting from the EINVF32 and EISQRTF32 instructions. Instead, check the operands for the following conditions (in code) before using each instruction:

Disregard the contents of the LUF and LVF flags by saving the flags to the stack before calling the instruction, and subsequently restoring the values of the flags once the instruction completes.

    MOV32                 *SP++,STF    ; Save off current status flags
    EISQRTF32/EINVF32                  ; Execute operation
    NOP                                ; Wait for operations to complete
    MOV32                 STF,*--SP    ; Restore previous status flags

If the PIE interrupts are tied to the LUF and LVF flags, disable the interrupts (at the PIE) before using either the EINVF32 or EISQRTF32 instruction. Check to see if the LUF and LVF flags are set; if they are, a variable can be set to indicate that a false LUF/LVF condition is detected. Clear the flags in the STF (FPU status flag) before re-enabling the interrupts.

Once the interrupts are reenabled at the PIE, the interrupt may occur (if the LUF/LVF interrupt lines were asserted by either of the two instructions) and execution branches to the Interrupt Service Routine (ISR). Check the flag to determine if a false condition has occurred; if it has, disregard the interrupt.

Do not clear the PIE IFR bits (that latch the LUF and LVF flags) directly because an interrupt event on the same PIE group (PIE group 12) may inadvertently be missed.

Here is an example:

_flag_LVFLUF_set    .usect ".ebss",2,1,1
    ...
    MOV32   *SP++,STF                       ; Save off current status flags
    ; Load the PieCtrlRegs page to the DP
    MOVW    DP, #_PieCtrlRegs.PIEIER12.all
    ; Zero out PIEIER12.7/8, i.e. disable LUF/LVF interrupts
    AND     @_PieCtrlRegs.PIEIER12.all, #0xFF3F
    EISQRTF32/EINVF32                       ; Execute operation
    MOVL    XAR3, #_flag_LVFLUF_set         ; Wait for operation to complete
    MOV32   *+XAR3[0], STF                  ; save STF to _flag_LVFLUF_set
    AND     *+XAR3[0], #0x3                 ; mask everything but LUF/LVF
    ; Clear Latched overflow, underflow flag
    SETFLG  LUF=0, LVF=0                    
    ; Re-enable PIEIER12.7/8, i.e. re-enable the LUF/LVF interrupts
    OR      @_PieCtrlRegs.PIEIER12.all, #0x00C0
    MOV32   STF,*--SP                       ; Restore previous status flags

In the ISR,

__interrupt void fpu32_luf_lvf_isr (void)
{
 // Check the flag for whether the LUF, LVF flags set by
  // either EISRTF32 or EINVF32
  if((flag_LVFLUF_set & 0x3U) != 0U)
  {
    //Reset flag
    flag_LVFLUF_set = 0U;
    // Do Nothing
  }
  else
  {
    //If flag_LVFLUF_set was not set then this interrupt
    // is the legitimate result of an overflow/underflow
    // from an FPU operation (not EISQRTF32/EINVF32)
    ...
    // Handle Overflow/Underflow condition
    ...
    ...
    ...
  }
  // Ack the interrupt and exit
}